Conventionally, an exhaust purifier which reduces NOx to nitrogen and water with a catalyst reactor in which a selective reduction NOx catalyst (SCR catalyst) is arranged and ammonia which is a reducing agent so as to decrease NOx (nitrogen oxide) included in exhaust gas from an internal combustion engine is known. The ammonia is generated by urea water injected to exhaust gas with high temperature and contacts the NOx catalyst so as to reduce NOx to nitrogen and water.
In the exhaust purifier, a material having a carrier of oxides, such as Ti including active ingredients, such as V and Cr which is formed to be a honeycomb structure in which many through holes are formed is used as the NOx catalyst of the catalyst reactor. According to the configuration, a contacting area is increased so as to promote a reduction reaction. On the other hand, when exhaust gas passes through the through holes, dust included in the exhaust gas may adhere to the NOx catalyst and close the holes so as to lower the reduction reaction. Accordingly, there is an art in which compressed air is injected to a NOx catalyst so as to remove dust adhering to the NOx catalyst and suppress adhesion of dust. For example, an art described in the Patent Literature 1 is so.
In a catalyst reactor (reactor) of an exhaust purifier (exhaust denitration device) described in the Patent Literature 1, if needed, compressed air is injected from an injection nozzle toward a NOx catalyst so as to hit dust adhering to the NOx catalyst, thereby removing the dust. Generally, the exhaust purifier is configured so that compressed air is injected periodically based on operation time of an engine. Namely, in the exhaust purifier, the compressed air is injected without considering an adhesion state of dust in the NOx catalyst. Accordingly, there is a problem that in the exhaust purifier, the compressed air is vainly consumed by unnecessary injection of the compressed air.